Field of the Invention
The present invention relates to a method for acquiring magnetic resonance (MR) signals from an examination object, and an associated MR apparatus.
Description of the Prior Art
For imaging using magnetic resonance installations (MR installations), it is normal practice to direct a two-dimensional excitation pulse into an examination object in order to excite magnetization of nuclear spins in the object. In the case of such an excitation pulse, the RF excitation pulse limits the region in which the magnetization is excited in two spatial directions, which are substantially perpendicular to each other, using two associated magnetic field gradients during the application of the 2D excitation pulse. The two-dimensional excitation pulse is described in Hardy C J et al., “Spatial localization in two dimensions using MR designer pulses”, J. Magn. Reson. 1989; 82:647-654. By contrast, in the case of a normal one-dimensional excitation pulse, the magnetization is limited in only one spatial direction by the RF pulse. The two-dimensional excitation pulse is used for imaging in a thin layer in the slice selection direction with a reduced field of view (FOV) in the phase coding direction, with the field of view in the phase decoding direction being reduced relative to the excitation with a one-dimensional excitation pulse. As a result of the reduced field of view, it is possible to improve the image quality by reducing the number of spatial coding steps that are required in the phase coding direction. This reduces the duration of the overall signal acquisition, which in turn reduces the artifacts caused by amplitude or phase changes. The amplitude changes are caused by T2 or T2* relaxation processes and the phase changes are produced by frequency differences due to the chemical shift and due to an inhomogeneity of the polarization field B0.
In the context of these imaging methods with reduced field of view, it is important to reduce unwanted signal components from regions outside the excited volume. Although the two-dimensional excitation works well for proton signals in water molecules, it is nonetheless difficult in many cases to suppress signal components from outside the desired volume of protons in fat. These unwanted signal components are added to the signals from the desired field of view and result in image artifacts.
Fat signals represent a particular problem, because the chemical shift of the protons in fat signals lies approximately 3.5 ppm (parts per million) away from the water protons. This corresponds to the difference in the resonance frequency of 224 Hz at 1.5 Tesla or 447 Hz at 3 Tesla. The two-dimensional excitation pulse usually has a frequency response that results in a non-ideal excitation pattern at a frequency offset which corresponds to the frequency of the fat resonance, and this results in the fat components being incorrectly localized in the desired field of view, even though these fat signal components lie outside the field of view.
Fat signal components are also disruptive in the case of certain measurements such as diffusion-weighted imaging methods, e.g. if the signal reduction of the water signal is greater than that of the fat signal, which increases the relative signal component of the fat.
In addition to the desire to suppress the fat signal outside the field of view, is often desirable to suppress the fat signal inside the field of view or within the region to be examined. This applies to EPI (echoplanar imaging) methods in particular, since the offset caused by the chemical shift produces an incorrect spatial coding between the fat signals and the water signals in the spatially coded image, and EPI methods are particularly sensitive to this.
In “DWI of the spinal cord with reduced FOV single-shot EPI”, Magn. Reson. Med. 2008, 60:468-473, Saritas et al. have shown that two-dimensional excitation pulses can be used in such a way as to produce a spatial offset in the slice selection direction between the examination region for the water protons and the examination region for the fat protons. It is then possible to generate an MR image on the basis of the water signal components alone, using a one-dimensional focusing pulse which selectively refocuses the magnetization during the spatial localization of the water protons. This method is effective for the suppression of fat signal components outside and inside the field of view, but this method has the disadvantage that the two-dimensional excitation in the slice selection direction is periodic, meaning that layers are excited periodically at intervals in the slice selection direction. This limits the number of layers that are picked up during a measurement, since the number of layers is limited by the periodicity of the excited slices.
Fat signal components can also be suppressed by further methods as follows:
Fat signals can be suppressed by frequency-selective fat excitation of the fat signal components over the whole examination object. However, this requires a high degree of homogeneity of the polarization field B0 and a homogeneous RF field for exciting the fat signal components. It is often not possible to generate the required homogeneity of the B0 field.
Also known is a so-called STIR method (Short Tau Inversion Recovery), in which a layer-selective inversion pulse is used to invert the magnetization. An inversion time T1 is then selected such that due to the T1 relaxation the magnetization of the fat signal component reaches the zero point with no magnetization along the direction of the polarization field B0. The imaging sequence is then started, whereby in the ideal case no fat signal components are produced. However, the inversion is applied for all regions of the examination object and has the disadvantage that it also reduces the signal-to-noise ratio of the water protons, since it involves a T1-dependent reduction of the signal in the B0 field direction. This technique is also restricted in that the properties of the layer-selective inversion, such as the layer profile and the bandwidth, must be compatible with the other RF pulses for the layer selection, which can mean limitations in the design of the imaging sequence and result in non-optimal fat suppression.
Also known are inversion imaging methods which use selective inversion depending on the chemical shift, wherein the inversion pulse is not used in the slice selection direction here, but as a frequency-selective pulse over the whole examination object. This likewise requires an often unachievable mobility of the polarization field B0.